Medical physics

What you'll learn

• Understand that a piezo-electric crystal changes shape when a p.d. is applied across it and generates an e.m.f. when its shape changes.
• Understand how ultrasound waves are generated and detected by a piezo-electric transducer.
• Understand how the reflection of pulses of ultrasound at boundaries between tissues can be used to obtain diagnostic information about internal structures.
• Define the specific acoustic impedance of a medium as Z = ρc.
• Use IR/I0 = (Z1 − Z2)2/(Z1 + Z2)2 for the intensity reflection coefficient of a boundary between two media.
• Recall and use I = I0e−μx for the attenuation of ultrasound in matter.
• Explain that X-rays are produced by electron bombardment of a metal target and calculate the minimum wavelength of X-rays produced from the accelerating p.d.
• Understand the use of X-rays in imaging internal body structures, including an understanding of the term contrast in X-ray imaging.
• Recall and use I = I0e−μx for the attenuation of X-rays in matter.
• Understand that computed tomography (CT) scanning produces a 3D image of an internal structure by first combining multiple X-ray images taken in the same section from different angles to obtain a 2D image of the section, then repeating this process along an axis and combining 2D images of multiple sections.
• Understand that a tracer is a substance containing radioactive nuclei that can be introduced into the body and is then absorbed by the tissue being studied.
• Recall that a tracer that decays by β+ decay is used in positron emission tomography (PET scanning).
• Understand that annihilation occurs when a particle interacts with its antiparticle and that mass-energy and momentum are conserved in the process.
• Explain that, in PET scanning, positrons emitted by the decay of the tracer annihilate when they react with electrons in the tissue, producing a pair of gamma-ray photons travelling in opposite directions.
• Calculate the energy of the gamma-ray photons emitted during the annihilation of an electron–positron pair.
• Understand that the gamma-ray photons from an annihilation event travel outside the body and can be detected, and an image of the tracer concentration in the tissue can be created by processing the arrival times of the gamma-ray photons.

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